To add a customer to a sale, go to the Point of Sale screen. Click the pink "Add a Customer" button on the left side of the screen.
Search for an existing account, or click "Create Customer" in the top right corner of the popup. This will open a new popup where you can insert the customer's name, email address, and other relevant information. Both Regular Consignor Accounts and Customer Accounts can be added to a sale. A Consignor by definition is an Account that owns items in the store. A Customer is an Account who has one or more purchases at a store. So, if an Account both owns items in the store and has purchases, it is both a Customer and Consignor.
Click "Create Account" when you're finished, and the new account will be applied to the sale.
NOTE: Customers can be applied to the sale at any point in the sale process.